Many items used in surgical suites are sterilized in advance of use and stored until needed. Bowls, pans, and the like are generally wrapped in two layers of cloth and sterilized in an autoclave, a device for generating saturated steam at high temperature, typically 250.degree. F. to 273.degree. F. (121.degree. C. to 134.degree. C.). Proper management of the sterile field in the suite requires verification of the sterility of the items before use. One present practice is to attach to the outside of the pack a steam sterilization monitor which indicates if temperature, humidity, and time conditions indicative of sterilization have occurred. One limitation of present monitors is that they cannot remotely detect whether the desired temperature and humidity conditions necessary for proper sterilization were present inside the cloth layers at the surface of the bowl or pan.
One present type of monitor, such as set forth in U.S. Pat No. 4,448,548, displays a change in color, or the wicking of a substance, or both, and thus requires a visual inspection. Similarly, monitors containing microorganisms, such as depicted in U.S. Pat. No. 4,591,566, require culturing the spores exposed to the sterilization process and cannot be remotely inspected. Monitors of these types are therefore less suitable for placement within the layers of cloth wrapped around the bowl or pan, as they cannot be inspected without removing the cloth layers. To prevent compromising the sterility of the contents, this inspection would have to be done immediately before use of the sterilized item in the suite. However, should the monitor show that the pack contents are not sterile, the surgical sterile field may be compromised, or at best, the surgical procedure is delayed while a replacement pack is obtained, if one is available. Therefore, a sterilization monitor which could be inspected without removing the sterile cloth would improve the management of surgical sterile fields as well as that of the inventory of the sterile items used in surgical suites.
A well-established technology to monitor the presence of objects in a given zone is employed in electromagnetic surveillance systems, such as that depicted in U.S. Pat. Re. No. 32,428. Typically, as set forth in that patent, a "marker" having certain magnetic properties so as to be capable of providing characteristic response when interrogated by an alternating magnetic field, is attached to the object to be monitored. When the marker passes through such an applied magnetic field in an "interrogation zone," the characteristic electromagnetic response is created and detected by appropriate circuitry. For applications in which a marker is to be permanently attached to an object, as in the spine of a book, and is desired to be deactivated, e.g., when the book has been properly checked out of a library, the surveillance system further comprises means for changing the response when desired.